1. Field of the Invention
The present invention relates to a transmission system of a base station and, more particularly, to an apparatus for improving peak-to-average ratio (PAR) characteristics of a transmission signal.
2. Description of the Background Art
A peak cancellation apparatus of a transmission unit of a base station in accordance with a conventional art will now be described with reference to the accompanying drawings.
FIG. 1 is a schematic block diagram of a transmission unit of a base station in accordance with a conventional art.
As shown in FIG. 1, the conventional base station transmission unit includes a parallel/serial converter 10 for converting parallel signals inputted from a plurality of channels to serial signals; a multiplier 20 for multiplying an output signal of the parallel/serial converter 10 and an inherent PN(Pseudo Noise) code of a base station and outputting a spread digital signal; a pulse shaping filter 30 for canceling an unnecessary signal component by limiting a band of the digital signal; D/A converter 40 for converting the digital signal outputted from the pulse shaping filter 30 into an analog signal; an I/Q modulation unit 50 for converting the analog signal into an intermediate frequency (IF) signal; an RF signal processing block 60 for converting the IF signal outputted from the I/Q modulation unit 50 into a transmittable radio frequency (RF) signal; and a power amplifier 70 for sufficiently amplifying power of the RF signal and transmitting it to a base station through an antenna.
The operation of the conventional base station transmission unit will now be described.
A pilot data and a plurality of traffic data outputted from a modem unit of a base station are coded in a coding unit (not shown) and divided into a component ‘I’ and a component ‘Q’, respectively. Passing the parallel/serial converter 10, the components ‘I’ are added to each other to form a channel ‘I’ and also the components ‘Q’ are added to each other to form a channel ‘Q’. Thereafter, the output signal of the parallel/serial converter 10 is transmitted to the multiplier 20 by way of certain circuits. In this respect, for explanations' sake, it is assumed that the output signal of the parallel/serial converter 10 is directly transmitted to the multiplier 20.
FIG. 2 is a drawing illustrating input and output waveforms of the parallel/serial converter 10.
As shown in FIG. 2, if there are three channels (a, b, c) and if data bits of the channels a, b and c are all ‘1’, the output signal of the parallel/serial converter 10 has a signal level of ‘3’ at the interval t1 and the interval t2.
If the channels are increased in number, a difference between the signal level at the two intervals t1 and t2 and an average signal level widens, and due to a high peak to average ratio (PAR), an intermodulation distortion (IMD) level of the base station output signal goes up. As a result, in order to accommodate a high peak signal, the transmission unit of the base station should use a large capacity power amplifier.
When the output signal of the parallel/serial converter 10 is transmitted to the multiplier 20, the multiplier 20 performs a spreading process to multiply the output signal of the parallel/serial converter 10 by the inherent PN code of a base station. An output signal of the multiplier 10 is transmitted to the pulse shaping filter 30. The digital signal inputted to the pulse shaping filter 30 contains an unnecessary frequency component as well as a desired frequency band and has a wider channel width than a desired band width. Thus, the pulse shaping filter 30 cancels the unnecessary signal component by limiting the band of the input signal.
The output signal of the pulse shaping filter 30 is converted into an analog signal in the digital/analog converter 40 and transmitted to the I/Q modulation unit 50. Upon receiving the analog signal, the I/Q modulation unit 50 converts it into an intermediate frequency (IF) signal.
The IF signal outputted from the I/Q modulation unit 50 is converted into a transmittable radio frequency signal while passing the RF signal processing block 60, and transmitted to the power amplifier 70.
The power amplifier 70 sufficiently amplifies the power of the received RF signal and wirelessly transmits it to a base station through the antenna.
Therefore, as mentioned above, the transmission unit of a base station in accordance with the conventional art has the following problems.
That is, in designing a system related to the PAR characteristics, the system should be designed to have a sufficient margin for a gain of the power amplifier, and in addition, use of the relatively many high-priced amplifier modules incurs unnecessary expense.
Moreover, the increase in the number of channels in the mobile communication base station system leads to occurrence of many peak signals in view of the channelization code, which inevitably degrades the performance of the overall system.
Furthermore, the high PAR causes an increase in the nonlinearity of the power amplifier and worsens the intermodulation characteristics of the base station output spectrum, degrading the transmission performance of the base station.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.